Transport Layer

1
Transport Layer

• Instructor Carey Williamson
• Office ICT 740
• Email carey_at_cpsc.ucalgary.ca
• Class Location ICT 122
• Lectures MWF 1200 1250
• Notes derived from Computer Networking A Top
  Down Approach, by Jim Kurose and Keith Ross,
  Addison-Wesley.
• Slides are adapted from the books companion Web
  site, with changes by Anirban Mahanti and Carey
  Williamson.

2
Chapter 3 Transport Layer

• learn about transport layer protocols in the
  Internet
• UDP connectionless transport
• TCP connection-oriented transport
• TCP congestion control

• Our goals
• understand principles behind transport layer
  services
• multiplexing and demultiplexing
• reliable data transfer
• flow control
• congestion control

3
Transport services and protocols

• provide logical communication between app
  processes running on different hosts
• transport protocols run in end systems
• send side breaks app messages into segments,
  passes to network layer
• rcv side reassembles segments into messages,
  passes to app layer
• more than one transport protocol available to
  apps
• Internet TCP and UDP

4
Transport vs. Network Layer

• transport layer logical communication between
  processes
• relies on, enhances, network layer services
• PDU Segment
• extends host-to-host communication to
  process-to-process communication
• network layer logical communication between
  hosts
• PDU Datagram
• Datagrams may be lost, duplicated, reordered in
  the Internet best effort service

5
TCP/IP Transport Layer Protocols

• reliable, in-order delivery (TCP)
• connection setup
• flow control
• congestion control
• unreliable, unordered delivery UDP
• no-frills extension of best-effort IP
• What does UDP provide in addition to IP?
• services not provided by IP (network layer)
• delay guarantees
• bandwidth guarantees

6
Multiplexing/Demultiplexing
HTTP
FTP
Telnet

• Use same communication channel between hosts for
  several logical communication processes
• How does Mux/DeMux work?
• Sockets doors between process host
• UDP socket (dest. IP, dest. Port)
• TCP socket (src. IP, src. port, dest. IP, dest.
  Port)

7
Connectionless demux

• UDP socket identified by two-tuple
• (dest IP address, dest port number)
• When host receives UDP segment
• checks destination port number in segment
• directs UDP segment to socket with that port
  number
• IP datagrams with different source IP addresses
  and/or source port numbers directed to same socket

8
Connection-oriented demux

• TCP socket identified by 4-tuple
• source IP address
• source port number
• dest IP address
• dest port number
• recv host uses all four values to direct segment
  to appropriate socket

• Server host may support many simultaneous TCP
  sockets
• each socket identified by its own 4-tuple
• Web servers have different sockets for each
  connecting client
• non-persistent HTTP will have different socket
  for each request

9
UDP User Datagram Protocol RFC 768

• no frills, bare bones Internet transport
  protocol
• best effort service, UDP segments may be
• lost
• delivered out of order to app
• Why use UDP?
• No connection establishment cost (critical for
  some applications, e.g., DNS)
• No connection state
• Small segment headers (only 8 bytes)
• Finer application control over data transmission

10
UDP Segment Structure

• often used for streaming multimedia apps
• loss tolerant
• rate sensitive
• other UDP uses
• DNS
• SNMP
• reliable transfer over UDP add reliability at
  application layer
• application-specific error recovery!

32 bits
source port
dest port
Length, in bytes of UDP segment, including header
checksum
length
Application data (message)
UDP segment format
11
UDP checksum

• Goal detect errors (e.g., flipped bits) in
  transmitted segment

• Sender
• treat segment contents as sequence of 16-bit
  integers
• checksum addition (1s complement sum) of
  segment contents
• sender puts checksum value into UDP checksum
  field

• Receiver
• compute checksum of received segment
• check if computed checksum equals checksum field
  value
• NO - error detected
• YES - no error detected. But maybe errors
  nonetheless? More later .

12
Internet Checksum Example

• Note When adding numbers, a carryout from the
  most significant bit needs to be added to the
  result
• Example add two 16-bit integers
• Weak error protection? Why is it useful?

1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1
0 1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0
1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1
1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1
0 0 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0
1 1
wraparound
sum
checksum